Effect on the displacement of a point source, in the equatorial plane of the black hole, with respect to the caustic (top) on the formation of images in the observer plane (bottom). The black astroid represents the primary caustic and the black circle represents the primary critical curve. We define the z′ and y′ axis whose origin corresponds to the middle of the astroid cross-section. The α and δ axis correspond to the observer’s sky axis, the origin is centered in the middle of the screen (because of the rotation of the black hole, the critical curve is shifted from the center of the screen).